Directive antenna



Oct. 28,1947. WWMEHER my 2,42 ,640

DIRECTIVE ANTENNA Filed'oct. l7, 1942 2 Sheets-Sheet l w. R B J.D.MALLETT wghw Oct. 28, 19-47. w, w MlEHER' -f 2,429,640

DIRECTIVE ANTENNA Fild Oct. 17, 1942 2 Sheets-Sheet 2 FIG. ll

INVENTORS:

' TTORNEY Patented Oct. 2 8, 1947 DIRECTIVE ANTENNA Walter W. Mieher,Mineola, and John D. Mallett,

Garden City, N. Y., assignors to Sperry Gyroscope Company, Inc.,Brooklyn, N. Y., a corporation of New York 7 Application October 17,1942, Serial No. 462,434

19 Claims. 1

This invention relates generally to the transmission and reception ofultra high frequency electromagnetic energy having a wavelength of theorder of one meter or less, and more specifi cally, to means forobtaining radiation beams.

and reception patterns which are highly directive and relatively veryfree of secondary lobes. I

A well known method for exciting a metallic paraboloid antenna is by theuse of a dipole terminating a concentric line, the dipole usually beinglocated at or near the effective focus of the paraboloid. Other methods,as disclosed in the copending application, Serial No. 429,494, entitledDirective antenna structure, filed February 4, 1942, in the names ofRobert J. Marshall, Wilmer L. Barrow, and Walter W. Mieher, include theuse of wave guides introduced substantially axially through the back ofthe paraboloid with reflecting means positioned in front of the mouth ofthe wave guide near the effective focus of the paraboloid to reflecttoward the paraboloid the energy emitted from the guide. Theaforementioned application also describes the use of wave guides orsmall electromagnetic horns of slight taper pointed substantiallyaxially into the paraboloid with their mouths positioned at or near theeffective foci of the paraboloids.

In these prior art devices, it has been difficult to evenly irradiatethe reflecting surface of the paraboloid due to the relatively largesize of the wave guides and reflectors themselves whereby they obstructthe radiated energy and also due to the fact that such combinations arerather broad sources so that all of the energy does not flow toward theparaboloid as from a point source. These and other factors tend tobroaden the primary beam and to introduce secondary lobe beams ofconsiderable magnitude, which are, as is well known, undesirable inaircraft instrument landing systems, target detection, and trackingsystems, and in kindred uses. I

It is therefore a principal object'of the present invention to providemeans for excitation of paraboloid radiators so that highly directivebeams of radiant energy, relatively free of secondary lobes, may beproduced. 7

A principal object is to provide means in a receiver paraboloid wherebythe reception pattern of the device is made highly directive andrelatively free of gain in directions other than that of the primarypattern.

Another object is to provide, in a. paraboloid, antenna and reflectormeans which behave as a point source or receiver. v

A furtherobject' is to, provide such wave guide means which lrradiatesevenly the entire reflective surface of a paraboloid.

Still another object of the invention lies in the provision of improvedimpedance matching means adapted for use with such Wave guide means.

These and other objects and advantages will become apparent from thespecification, taken in connection with the accompanying drawings,wherein the invention is embodied in concrete form.

In the drawings,

Fig. 1 is a perspective View of a preferred form of the presentinvention.

Fig. 2 is a partial cross-section view of a detail of Fig. 1. v

Fig. 3 is a perspective view of a detail of Fig. 1.

Fig. 4 is an explanatory graph.

Fig. 5 is a schematic diagram of the reflector and radiator.

Fig. 6 is a partial cross-section view of an alternative form of thedevice shown in Fig. 2.

Fig. 7 is a cross-section view of an alternative form of a portion ofFig. 6.

Fig. 8 is an explanatory schematic diagram.

Fig. 9 is a partial cross-section view of an alternative form of aportion of Figs. 2 or 6.

Fig. 10 is a partial cross-section view of an alternative form of theinvention.

Figs. 11 and 12 are perspective views of modified forms of theinvention.

Figs. 13, 14, and 15 are alternative forms of details of the invention.

Similar characters of reference are used throughout the figures toindicate corresponding parts.

In Figs. 1, 2 and 3 is illustrated as a preferred form of the presentinvention an ultra high frequency paraboloid antenna capable ofproducing a highlydirective beam of radiation, as shown in Fig. 4, withsecondary lobes containing a minimum amount of energy. Other shapes ofconcave electromagnetic wave mirrors or reflectors may be used, e. g.,spherical, but for simplicity in description those having surfaces ofrevolution formed by rotating parabolas and other similar shapes aboutthe axis of symmetry herein will be referred to comprehensively asparaboloids. A paraboloid l preferably having its axial dimensionsubstantially equal to its focal length, is irradiated with ultra highfrequency energy from an antenna structure 3, which is fed in' anyorthodox manner from a wave guide or energyconducting pipe 2. Referringto Figs. 2 and 3, antenna structure 3 is seen to consist of arectangular hollow wave guide 2 joined to a tapered 3 metallic portionor transition member 4 in which is inserted a dielectric guide 5, inthis case shown as a guide formed of material having a relatively highdielectric constant. The dielectric guide 5 and wave guide 2 areprovided with discontinuous sections forming an impedance matchingtransformer T.

The use of a suitable dielectric material, typically a commerciallyavailable thermo-plastic composition known as Polystyrene, permits thecritical dimensions of the wave guide 2 to be reduced in the region ofthe antenna structure, thereby concentrating the beam energy to beradiated by the rod 6 and minimizing obstructions to the wave reflectedfrom the paraboloid I. Reduction of the cross-sectional dimensions ofthe guide in the impedance matching section T extending between the waveguide 2 and the radiator 6 also minimizes the obstruction to the energyradiated toward the paraboloid.

Extending into hollow guide 2 is a rectangular extension of dielectricmaterial whose crosssection may be somewhat reduced from that of themain portion of guide '5. The cross-sectional area of portion 8 isadjusted experimentally so that the following relation obtains:

where Z1 is the characteristic impedance of hollow guide 2, Z2 is thatof the section containing dielectric portion 8, and Z; is that portionof 4 of the dielectric guide 5. The dielectric matching section isadjusted in length V by experi ment, its length being substantially afourth of the average of the wavelength of the electromagnetic energywithin the guides.

It is seen that a series of such steps, as seen at 8', 8" in Fig. 7, maybe used as an impedance transforming means between the guides 2 and 5,as disclosed in copending application Serial No. 437,004, entitled Waveguide construction, filed March 31, 1942, in the names of Montgomery H.Johnson, William H. Ratlifi, and William W. Hansen. It is therein shownthat if the co efficients of (:t-I-l)", known as binomial coeificients,are used in describe the increments in the logarithm of thecharacteristic impedance of successive quarter-wavelength sections ofwave guide making up an impedance matching transformer between waveguides of dilferent characteristic impedance, then, as n is increased,the frequency range over which such a wave guide transformer means isuseful is increased.

The dielectric guide 5 projects on past the end of tapered metallicportion 4, and has transversely extending through such projectingportion adjacent to the principal focus of the paraboloid a roundconducting rod or antenna 6, which extends out of the guide equally oneach side of the guide. Rod 6 is approximately a'halfwavelength long andacts as the chief source of radiation to irradiate the paraboloidreflecting surface. As seen in Fig. 8, rod 6 is excited as shown by thegraph I0, so that voltage anti-nodes appear at its opposite'ends. At theend of guide 5 is placed reflector plate or element I, which isdimensioned so as to act not only to reflect energy toward paraboloid I,but also, in conjunction with radiating rod IE, to act to cancel out allenergy traveling in the direction of the arrow "I3 of Fig. 2 directlyfrom rod 6. The diameter of rod 6 is not critical, and as seen at 6 inFig. 9, is also not critical as to shape, although it preferably is madeeffectively a half-Wavelength long. The length of rod 6 also has beenfound to be not critical, since excellent results were obtained with arod whose actual length was slightly in excess of one-half wavelength infree space. In general, shapes of rod 6 having larger diameter havelower loss and are less sharply resonant, thereby having more constantgain over larger frequency ranges.

The preferred geometrical relationship of the parts may be discussed byreferring to Fig. 5. In general, reflector element I is placed in theplane of the front of the paraboloid, and is made thin-walled andsquare, of dimensions (S) about equal to one wavelength. Substantiallyone quarter-wave (P) behind reflector element 1 is placed the half wavelong radiating rod or antenna G. -A very short distance (Q) furtheralong the dielectric guide 5 begins the substantially quarter-wave long(R) tapered portion 4, which joins directly to hollow rectangular guide2. The

distance Q is experimentally adjusted to make the distance from rod 6 tothe shoulder I I such that it transforms the impedance looking in thedirection of hollow guide 2 into a pure resistance. For example, for aparaboloid of diameter 30 cm., focus 7.5 cm. and a wavelength ofsubstantially 3 cm. and a dielectric guide of substance knowncommercially as Polystyrene, the following set of dimensions has yieldedgood results: dielectric guide 0.62 cm. by 1.45 cm.; S. 2.37 cm. square;P, 0.82 cm.; Q, 0.32 cm.; and R, 0.79 cm., where the antenna 6 is 0.24cm. in diameter and 1.64 cm. in length. A structure made according tothe foregoing provides a highly directive pattern of the general shapeshown in Fig. 4, which pattern indicates the results obtained durinexperimental tests.

In general, however, the length R of tapered portion 4 may be any oddnumber of quarterwavelengths and is chosen to be of suflicient taper soas not to cast a shadow on the reflecting surface of paraboloid I; i.e., so that substantially none of the reflecting surface of paraboloid Iis hidden from the effective source, at or adjacent to the rod 6.Tapered portion I is introduced so that the transformation means betweenhollow guide 2 and dielectric guide 5 can be located as close aspossible to radiating rod 6, whereby dielectric guide 5 is of minimumlength, and attenuation is minimized.

The discontinuities in the waveguides'may produce undesirablereflections toward the energy source. This difllculty might be overcomeby adjusting the distances R and V until reflections from thediscontinuity at I I are neutralized by reflections from the outer endregion of the guide 5. Additional slight adjustment may be made byadjusting'the dimension Q, 'or t'elescoplng the entire guide in or outof'pipe 2,

If desired, as shown in Fig. 6, tapered portion 4 can be eliminated andthe impedance matching transformer T and shoulder 26 formed by theabrupt reduction indimensions-of the casing can belocated behind theparaboloid I in which case the shoulder cannot produce undesirableradiation. 'The 'metallicenvelope ofhollow guide 2 is decreased in sizeat point I I where the matching transformer T begins and extends intothe paraboloid I, the decrease in'si'ze of guide 5 being proportional toWhere e is the dielectric constant of the dielectric material. Thedistance between point II and radiating 'rodB is again made 'su'chthatthe imposition by the dielectric material.

pedance seen looking in the direction of hollow guide 2 is purelyresistive. It is obvious that the transformer described in Fig. 7 can besubstituted for the one shown in Fig. 6 as can any other well known typeof wave guide impedance matching means be substituted in any of theembodiments shown in the drawings.

Fig. 10 discloses a modified form of construction wherein the casing 12of the dielectric filled wave guide 5' extends to the reflector 1. Amodifled antenna rod 20 having spherical caps l 9, [9, extends throughapertures 2|, 2| formed in opposite sides of the casing I 2, the rodbeing held in The caps l9, [9' provide some measure of capacitiveantenna coupling and permit the use of a shorter antenna rod, therebyreducing the size of the radiator more nearly to a desirable pointsource of energy.

Figs. 11 through 14 illustrate embodiments of the present inventionwherein use is made of a reflector plate having at least a portionforming a partial surface of revolution about an axis generally parallelto the direction of the lines of electric force Within the wave guide 2and to the axis of radiator 6. Such reflector plates are located withtheir convex surface facing the paraboloid and may be substituted forthe plane plate 1 shown in the preceding views of the drawing,

Figs. 11 and 12 illustrate in perspective, antenna structures 3incorporating such a modified form of reflector plate. In Fig. 11 apreferably flat reflector plate I4 is provided on its reflecting surfacewith oppositely positioned half conical sections 15, I5 disposed along acommon vertical axis with their apices meeting at a common point locatedon the longitudinal axis of the guide 5. The curved surfaces of theconical sec-- tions l5, l5 form an antenna array composed of an infinitenumber of individual rods extending through the apex and betweenopposite bases of the conical members. For optimum performance, thedistance between bases should be of the order of a multiple of one-halfwavelength to increase reflection of energy in the range of operatingfrequencies, and the surfaces should extend through approximately half arevolution. As will be observed in the drawings, the axis of the conicalmembers l5, l5 extends perpendicularly to the broader sides of thewaveguide 2, and to the waveguide axis, and parallel to the electriclines and to the axis of the rod 6. Better results Were obtained with areflector element having a surface curved about a single axis, asdescribed, than were obtained with spherical or other convex surfaces ofcompound curvature. If desired, the dielectric guide may be dispensedwith and, as shown in Fig. 12, two arms [1, ll of dielectric materialmay be used to support the plate I 4.

Figs. 13 and 14 illustrate reflectors which likewise curve about an axisin a single plane, e. g., vertical, for use with the guide 2 positionedas shown in Fig. 11. In Fig. 13, the reflector plate comprises a metalmember 22 having coplanar end portions 24 and an intermediate convexportion 23 preferably comprising a half section of a cylinder, thediameter of which is of the order of the width of the wave guide 2. Threflector 22' shown in Fig. 14 differs from that shown in Fig. 13 inthat the coplanar portions 24 are replaced by convex surface portions24' having a larger radius of curvature about a vertical axis thanportion 23. It appears that reflectors with surfaces having simplecurvatures, e. g., revolved order of a multiple of one-half wavelength.In

use, the plates are positioned with their convex sides facing theparaboloid.

In Fig. 15, a reflector plate 25 comprises a plane plate having top andbottom edges 25 that approach one another from a maximum separationdistance equivalent to a wavelength or other multiple of halfwavelengths. The plate 25 therefore comprises an antenna array formed ofan infinite number of vertical rods having natural oscillationfrequencies in the operating range.

The central portion resonates with the principal frequency used in thesystem and successively adjoining portions have natural frequencies thatare higher as the distance between the opposite edges 25' decreases. Asshown in Fig. 15, the edges 25' may have a shape determined by a radiusof curvature equal to the maximum separation distance, e. g., onewavelength, and swinging about a median line.

It is evident to one skilled in the art that components herein disclosedcan be interchangeably used; and, although the use of the presentinvention has been described largely for the transmission of highlydirective beams of electromagnetic energy, that the invention is equallyuseful as a receiver antenna.

As many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

What is claimed is:

1. A directive antenna system comprising a paraboloid reflector, a waveguide comprising a pipe extending axially through said paraboloidreflector and including a dielectric guide of reduced cross-sectionhaving an end within said reflector, a radiating rod extending throughsaid dielectric guide at approximately the focus of said paraboloidreflector, and a reflector element at the end of said dielectric guide.

2. A directive antenna system comprising a paraboloid reflector, a waveguide comprising a pipe extending axially through said reflector, saidguide also including a dielectric guide section, a rod antenna extendingthrough said dielectric guide section at approximately the focus of saidreflector, and a reflector element at said end of said dielectric guideremote from said pipe.

3. A directive antenna system comprising a wave guide comprising a pipeand including a dielectric guide of reduced cross-section extending fromthe end of said pipe, a rod antenna extending through said dielectricguide, and a refiector element at the end of said dielectric guideremote from said pipe.

4. A directive antenna system comprising a wave guide including a pipehavin an open end and a dielectric member extending from said pipe, anda rod antenna supported by said dielectric member and spaced from saidend.

5. A directive antenna system comprising a wave guide and a rod antennamounted rigidly 7 conducting means coupling said wave guide and rod.

6. A directive antenna system comprising a concave electromagnetic wavemirror; a wave guide comprising a pipe extending through said mirror, a,dielectric guide of smaller cross-section than said pipe, and a tubulartransition member surrounding said dielectric guide and connecting withsaid pipe; at radiating rod located adjacent to. the principal focus ofsaid mirror and extending through said dielectric guide substantiallyperpendicularly to the axis of said pipe; and a reflector element at theend of said dielectric guide remote from said pipe.

'7. A directive antenna as claimed in claim 6, wherein said tubulartransition member has tapered outer surfaces sloping from the walls ofsaid pipe to. said dielectric guide.

8. A directive antenna system comprising a concave electromagnetic wavemirror; a wave guide comprising a pipe extending through said mirror, adielectric guide of smaller cross-section than said pipe, and a tubulartransition member surrounding said dielectric guide and connecting withthe end of said pipe; a radiating rod extending throughsaid dielectricguide at a point adjacent to the focus of said mirror, and a reflectorelement at the end of said dielectric guide remote from said pipe, saidtransition member extending from said pipe for a distance electricallyneutralizing reflected energy from points adjacent to the end of saiddielectric guide remote from said pipe.

9. An electromagnetic wave radiator comprising an elongated wave guideadapted to propagate high frequency electromagnetic energy therealongand having an open end, and a radiating rod disposed perpendicularly tothe axis of said guide in front of said open end, said rod beingsubstantially bigger in cross-section at its middle than at its ends. a

10. An electromagnetic wave antenna comprising a wave guide, a rodantenna disposed perpendicularly to the axis of said guide and adjacentto but spaced from the end of said guide, and electromagneticenergy-conducting means coupled to said Wave guide and supporting saidantenna rigidly with respect to said guide.

11. An antenna as in claim 10, wherein said rod antenna is substantiallyone-quarter wavelength long at the operating frequency.

12. A radiator as claimed in claim 9 wherein said radiator issubstantially elliptical in shape.

13'. An electromagnetic wave radiator compri ing a wave guide having anouter conductive casing, and a radiating rod extending through butinsulated from said casing, said rod having capacitive coupling membersthereon spaced from said casing.

14. A radiator as claimed in claim 13' wherein said: coupling memberscomprise spheroids adjacent to the extremities of said rod.

15. A directive antenna system comprising a concave electromagnetic wavemirror; a wave guide comprising an electrically conductive pipeextending axially through said mirror and a dielectric guide of smallercross-section than said pipe and having an end extending from said pipeWithin said mirror; a radiating rod extending through said dielectricguide at a point adjacent to the focus of said mirror, a reflectorelement at the end of said dielectric guide, an electrically concaveelectromagnetic wave mirror;

conductive transition member having tapered walls extending from saidpipe to said dielectric guide, said radiating rod being spaced alongsaid dielectric guide from the end of said member a distance sufficientto balance out undesirable refiection of radiant energy from said end ofsaid dielectric guide.

16. A directive antenna system comprising a concave electromagnetic wavemirror, a wave guide comprising an electrically conductive pipeextending axially through said mirror, a dielectric guide of smallercross-section than said pipe and having an end within said mirror, aradiating rod extending through said dielectric guide at a pointadjacent to the focus of said mirror, a reflector element, at the end ofsaid dielectric guide, an electrically conductive transition membercomprising a tube of smaller cross-sectional area than said pipe andbeing filled with dielectric material and interconnecting said pipe andsaid guide, the length of said member and the spacing of said radiatingrod from the end of said member adjacent said rod being such as tocancel out reflections from the end of said guide.

17. A directive antenna system comprising a a wave guide comprising anelectrically conductive pipe extending axially through said mirror, adielectric filled guide and a dielectric guide f. smaller cross-sectionthan said pipe extending from said pipe and having an end within saidmirror; a radiating rod extending through said dielectric guide at apoint adjacent to the focus of said mirror, a reflector element at theend of. said dielectric guide, and an impedance matching transformeradapted to couple said pipe and said dilectric guide.

18. A directive antenna system comprising a hollow metal pipe having anopen end, a dielectric member extending coaxially outwardly from saidend and fixed to said pipe, an antenna member supported by saiddielectric member in spaced relation to said open end, and a reflectormember also supported by said dielectric member and spaced from saidantenna member on the side thereof opposite; said. open end.

19. A directive antenna system comprising a hollow metallic pipe. havingan open end, a dielectric. member mounted coaxially with respect to saidpipe and extending outwardly from said open end, and an antenna membersupported by said dielectric member in spaced relation to said open end.

WALTER W. MIEHER. JOHN D. MALLETT.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,206,923 SouthWorth July 9, 19402,197,122 Bowen Apr. 16, 1940 2,207,845 Wolfi "-1 July 16, 19402,283,935 King May 26, 1942 2,028,498 Clavier Jan. 21, 1936 OTHERREFERENCES She-rt wave a Television, April1938, pages 669, 706 and= 707.

